VoNR Mobile Originated Call Flow | Voice Over NR Call Setup Tutorial

Introduction

A VoNR Mobile Originated (MO) Call occurs when a User Equipment (UE) initiates a voice call using the 5G NR radio access network.

In this scenario, the UE sends a SIP INVITE message to the IMS network, which then routes the call to the destination user.

Unlike fallback-based voice solutions, VoNR calls are handled entirely within the 5G Standalone network and IMS architecture.

The procedure is defined by the 3rd Generation Partnership Project specifications:

• 3GPP TS 23.501
• 3GPP TS 23.502
• 3GPP TS 24.229

Network Elements Involved

Several network components participate in the VoNR MO call setup.

VoNR Mobile Originated Call Signaling Flow

UE        gNB        AMF        IMS        UE-B
 |         |          |          |          |
 |---SIP INVITE--------------------------->|
 |         |          |          |          |
 |<------100 Trying------------------------|
 |<------180 Ringing-----------------------|
 |<------200 OK----------------------------|
 |---------ACK---------------------------->|
 |=========== RTP Voice Media ============|

Step-by-Step VoNR MO Call Procedure

Step 1: SIP INVITE Sent by UE

When the user dials a phone number, the UE generates a SIP INVITE message.

The INVITE contains:

• caller identity
• destination number
• media capabilities (codec list)
• session description (SDP)

The SIP INVITE is sent through the IMS PDU session.

Important parameters to check

Step 2: SIP Routing Through IMS

The P-CSCF forwards the SIP INVITE to the S-CSCF.

The S-CSCF performs:

• subscriber authentication
• call routing
• session control

The call is then routed to the destination network.

Important parameters to check

Step 3: Ringing Response

Once the destination UE receives the INVITE, the network returns signaling responses.

Typical responses include:

These responses are forwarded back to the calling UE.

Step 4: Call Acceptance

When the destination user answers the call, the destination UE sends 200 OK.

The calling UE then sends an ACK message.

After this exchange, the voice session becomes active.

Step 5: Voice Media Transmission

Once the call is established, voice media is transmitted using RTP (Real-Time Transport Protocol).

Voice packets flow between the two UEs via the UPF in the 5G Core network.

Typical codec options include:

EVS provides high-definition voice quality in VoNR networks.

QoS Configuration for VoNR Voice Calls

VoNR voice calls use a dedicated QoS flow to ensure low latency.

This ensures high reliability and minimal delay for voice packets.

Troubleshooting VoNR MO Call Issues

Call Setup Failure

Possible causes:

• SIP INVITE rejected
• IMS routing failure
• subscriber restrictions

Codec Negotiation Failure

Possible reasons:

• incompatible codecs
• incorrect SDP parameters

Poor Voice Quality

Possible causes:

• incorrect QoS configuration
• packet loss in user plane
• network congestion

Summary

A VoNR Mobile Originated Call allows a UE to initiate voice communication directly over the 5G NR network.

The process includes:

1. UE sending SIP INVITE
2. IMS routing the call
3. destination device ringing
4. call acceptance with 200 OK
5. voice media transmission using RTP

This procedure enables native 5G voice services with lower latency and improved audio quality.